• Abarenicola Wells, 1959
• Abarenicola affinis (Ashworth, 1903)
• Abarenicola affinis affinis (Ashworth, 1902) represented as Abarenicola affinis (Ashworth, 1903)
• Abarenicola affinis africana Wells, 1963
• Abarenicola affinis chilensis Wells, 1963
• Abarenicola affinis clarki Wells, 1963
• Abarenicola assimilis (Ehlers, 1897)
• Abarenicola assimilis assimilis (Ehlers, 1897) accepted as Abarenicola assimilis (Ehlers, 1897)
• Abarenicola assimilis brevior Wells, 1963 accepted as Abarenicola brevior Wells, 1963
• Abarenicola assimilis devia Wells, 1963 accepted as Abarenicola devia Wells, 1963
• Abarenicola assimilis haswelli Wells, 1963 accepted as Abarenicola haswelli Wells, 1963
• Abarenicola assimilis insularum Wells, 1963
• Abarenicola brevior Wells, 1963
• Abarenicola claparedei (Levinsen, 1884) accepted as Abarenicola claparedi (Levinsen, 1884)
• Abarenicola claparedi (Levinsen, 1884)
• Abarenicola claparedi oceanica (Healy & Wells, 1959)
• Abarenicola claparedi vagabunda Healy & Wells, 1959
• Abarenicola claparedii [auctt.] accepted as Abarenicola claparedi (Levinsen, 1884)
• Abarenicola devia Wells, 1963
• Abarenicola gilchristi Wells, 1963
• Abarenicola haswelli Wells, 1963
• Abarenicola insularum Wells, 1963
• Abarenicola pacifica Healy & Wells, 1959
• Abarenicola pusilla (Quatrefages, 1866)
• Abarenicola vagabunda Healy & Wells, 1959 accepted as Abarenicola claparedi vagabunda Healy & Wells, 1959
• Abarenicola vagabunda oceanica Healy & Wells, 1959 accepted as Abarenicola claparedi oceanica (Healy & Wells, 1959)
• Abderos Schüller & Jirkov, 2013
• Abderos minotaurus Schüller & Jirkov, 2013
• Aberranta Hartman, 1965
• Aberranta banyulensis Mackie, Pleijel & Rouse, 2005
• Aberranta enigmatica Hartman, 1965
• Aberranta palpata Wolf, 1987
• Aberranta sulcata Mackie, Pleijel & Rouse, 2005
• Aberrantidae Wolf, 1987
• Abyssarya Bonifácio & Menot, 2018
• Abyssarya acus Bonifácio & Menot, 2018
• Abyssinoe [auct.] accepted as Abyssoninoe Orensanz, 1990
• Abyssinoe scopa [auct. lapsus] accepted as Abyssoninoe scopa (Fauchald, 1974)
• Abyssocapitella Buzhinskaja & Smirnov, 2000
• Abyssocapitella commensalis Buzhinskaja & Smirnov, 2000
• Abyssoclymene Hartman, 1967
• Abyssoclymene annularis Hartman, 1967
• Abyssoninoe Orensanz, 1990
• Abyssoninoe abyssorum (McIntosh, 1885)
• Abyssoninoe bidentata D'Alessandro, Cosentino, Giacobbe, Andaloro & Romeo, 2014
• Abyssoninoe galatheae (Knox & Green, 1972)
• Abyssoninoe hibernica (McIntosh, 1903)
• Abyssoninoe orensanzi Hartmann-Schröder & Rosenfeldt, 1992
• Abyssoninoe phuketensis Oug, 2002
• Abyssoninoe scopa (Fauchald, 1974)
• Abyssoninoe scopa aequilobata (Winsnes, 1981) accepted as Abyssoninoe hibernica (McIntosh, 1903)
• Abyssoninoe winsnesae Frame, 1992
• Acanthicolepis McIntosh, 1900
• Acanthicolepis asperrima (M. Sars, 1861)
• Acanthicolepis cousteaui Laubier, 1961 accepted as Acanthicolepis asperrima (M. Sars, 1861)
• Acanthicolepis equitis (McIntosh, 1885) accepted as Harmothoe globifera (Sars G.O., 1873)
• Acanthicolepis longicirrata Treadwell, 1941 accepted as Eunoe purpurea Treadwell, 1936
• Acanthicolepis zibrowii Barnich & Fiege, 2010
• Acanthobdella Grube, 1851
• Acanthobdella peledina Grube, 1851
• Acanthobdellidea
• Acanthobonellia Fisher, 1948
• Acanthobonellia miyajimai (Ikeda, 1904)
• Acanthobonellia pirotanensis Jose, 1964
• Acanthobonellia rollandoe Menon, DattaGupta & Johnson, 1964
• Acanthobonellia vulgaris Menon, DattaGupta & Johnson, 1964 accepted as Acanthobonellia pirotanensis Jose, 1964
• Acanthobonelliinae DattaGupta, 1976 accepted as Bonelliidae Lacaze-Duthiers, 1858
• Acanthodrilidae Claus, 1880
• Acanthohamingia Ikeda, 1910
• Acanthohamingia ijimai (Ikeda, 1908)
• Acanthohamingia paradola Fisher, 1946 accepted as Amalosoma paradolum (Fisher, 1946)
• Acanthohamingia paradolum accepted as Acanthohamingia paradola Fisher, 1946 accepted as Amalosoma paradolum (Fisher, 1946)
• Acanthohamingia shiplei Ikeda, 1910
• Acanthohaminginae DattaGupta, 1976 accepted as Bonelliidae Lacaze-Duthiers, 1858
• Acanthopale San Martín, 1986
• Acanthopale perkinsi San Martín, 1986
• Acerrotrupa Yu & Wang, 1981 †
• Acerrotrupa aggregata Yu & Wang, 1981 †
• Acesta Strelzov, 1973 accepted as Aricidea (Acmira) Hartley, 1981
• Achaetobonellia Fisher, 1953
• Achaetobonellia maculata Fisher, 1953
• Achaetobonellia ricei Biseswar, 2005
• Acholoe Claparède, 1870
• Acholoe astericola (Delle Chiaje, 1841) accepted as Acholoe squamosa (Delle Chiaje, 1827)
• Acholoe orbiculata Treadwell, 1921 accepted as Acholoe squamosa (Delle Chiaje, 1827)
• Acholoe pulchra (Johnson, 1897) accepted as Arctonoe pulchra (Johnson, 1897)
• Acholoe squamosa (Delle Chiaje, 1827)
• Acholoinae Pettibone, 1996 accepted as Polynoinae Kinberg, 1856
• Acicularia Langerhans, 1877
• Acicularia virchowii Langerhans, 1877 accepted as Sagitella kowalewskii Wagner, 1872
• Aciculaspio Blake & Ramey-Balci, 2020
• Aciculaspio anaximanderi Blake & Ramey-Balci, 2020
• Aciculata accepted as Errantia
• Aciculomarphysa Hartmann-Schröder in Hartmann-Schröder & Zibrowius, 1998
• Aciculomarphysa comes Hartmann-Schröder in Hartmann-Schröder & Zibrowius, 1998
• Aclymene Buzhinskaja, 1995
• Aclymene gesae Buzhinskaja, 1995
• Acmira Hartley, 1981 accepted as Aricidea (Acmira) Hartley, 1981
• Acmira catherinae (Laubier, 1967) represented as Aricidea (Acmira) catherinae Laubier, 1967
• Acmira cerrutii (Laubier, 1966) represented as Aricidea (Acmira) cerrutii Laubier, 1966